Given a user query, an information retrieval (IR) system, for example a search engine, produces a ranked list of results that are most likely to satisfy a users needs and in the example of a search engine, the results may be web pages or images. In another example, the IR system may be an online shopping service and in this case the results may be suggestions of other items to buy based on a single purchase made by a user, or the IR system may be an online advertising system where the results are the advertisements displayed to a user.
To improve the relevance of the results provided, an IR system may tailor results based on the preferences of other users. For example in online shopping a user may be told ‘users who bought X also bought Y’. In search applications, users may re-rank the results (or the revised rank may be inferred based on whether a user clicks on a particular link or not) and this information may be used to improve results provided to other users. There are, however, security considerations with this sharing of data. In particular, the privacy of a user (i.e. ensuring that other users do not learn about a user's exact search patterns or retrieved documents) and the quality of the shared data (i.e. ensuring that the system cannot be overly influenced by a user that maliciously injects information to manipulate the results provided).
Some IR systems use social networks to create a personalized social search which determines the ranking of documents based on the preferences within a group of friends. An example method of personalized social search allows users to designate search mates with whom they share their search preferences. Privacy is maintained by enabling a user to specify who can see their preference data and further privacy features can be provided through the ability to perform private searches, delete previous searches or through enabling opting-in or opting-out of personalized searching.
The embodiments described below are not limited to implementations which solve any or all of the disadvantages of known preference propagation systems.